Dry powder compositions for preparing paint strippers

ABSTRACT

Alkaline paint stripper compositions contain either a fibrous material or a combination of a xanthan gum and hectorite clay so as to provide a pellable skin over a painted surface.

RELATED APPLICATIONS

This is a division of our prior, copending application Ser. No. 343,156,filed Jan. 27, 1982, which is a continuation-in-part of our prior,copending application Ser. No. 212,079, filed Dec. 1, 1980, nowabandoned.

(a) Field of the Invention

This invention relates to paint strippers, and in particular to paintstripping compositions containing an alkaline material.

(b) Description of the Prior Art

Tremain et al. U.S. Pat. No. 1,714,530 describes an alkaline paint andvarnish remover which is constituted in a solid form containing, interalia, starch as a thickening agent. The starch is stated to serve thepurpose of holding the composition together and to cause it to adhere toa surface to which it is applied. The solid compositions are preparedfor use by mixing with water. The aqueous mixture is then applied to apainted or varnished surface, and when the paint or varnish issufficiently soft, the surface is then flushed off with water.

Bond et al. U.S. Pat. No. 2,447,052 describes an alkaline paint removerintended for use in stripping paint from vertical steel surfaces. Thealkaline compositions contain a gelling agent, such as bentonite or"hydrous oxide gels of iron and cerium and organic gelling agents suchas starch, proteins and various gums", which give the compositionssufficient consistency to enable them to adhere to vertical surfaces. Inuse the compositions are spread on the painted surface by brushing orspraying, and after the paint has softened, the surface is washed offwith water.

Gansser U.S. Pat. No. 3,615,825 describes alkaline paint strippingcompositions containing alkali and an alkoxy-alkylamine in an aqueousmedium. Optionally the compositions may contain a thickening agent suchas starches, flours, methyl cellulose or glue.

Brainard U.S. Pat. No. 1,657,147 discloses non-caustic varnish and paintremover compositions containing carbonate, i.e. sodium carbonate, andsodium silicate as a gelling and thickening agent.

Ellis U.S. Pat. No. 2,029,992 discloses a liquid finish removerconsisting of a solution of paraffin wax in benzene and acetone. In usethe solution is spread over the painted surface to be cleaned, andsawdust is sprinkled over the surface in order to form an absorbent masswhich is more readily removed than the liquid slush alone.

SUMMARY OF THE INVENTION

This invention relates to paint stripping compositions which are soconstituted that they form a film or peelable skin over a paintedsurface which facilitates removal of paint from the surface. Thecompositions are thus particularly valuable in removing paint fromvertical or uneven surfaces or from surfaces having complex shapes suchas furniture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

When redecorating it is frequently desirable to remove old layers ofpaint, either to provide a good basis for new paint or to expose theunderlying surface. Traditionally, a blow-torch has been employed tosoften the old paint which is removed with a scraper. However, thisrequires considerable skill, and even experienced decorators can scorchthe underlying surface. This may not be a serious drawback when thesurface is to be repainted, but where, for example, a cleaned woodensurface is to be left unpainted, scorch marks are unsightly.

The amateur decorator therefore generally avoids using a blow-torch, andinstead relies on chemical paint strippers. In general, such paintstrippers are applied to the painted surface, left for a time in orderto allow the paint to become soft, and then paint and stripper areremoved together with a scraper. Paint strippers fall into two broadcategories: organic solvent-based strippers and aqueous, alkalinestrippers.

Organic solvents are used as the basis for the first category ofconventional strippers, often in conjunction with an alkali. These arevery effective but extremely unpleasant to use because of the presenceof the solvent, and the "thin" nature of the composition which leads todrips and runs.

In place of the organic vehicle, water has been used to form aqueouscompositions which usually contain an alkali as the activepaint-stripping ingredient. Again the "thin" nature of such compositionsmakes them inconvenient to use, and also the tendency of the compositionto evaporate and to run off vertical surfaces means that the activeingredient is not in contact with the paint long enough to result inefficient stripping.

A recent improvement in aqueous strippers has been obtained by includinga gellant and a filler such as chalk in the composition to give a pastyconsistency. This makes application of the stripper to a painted surfacemuch easier and removes the problem of dripping, while removal of thestripper and softened paint from flat surfaces is greatly facilitatedsince it can be virtually lifted off the surface using a paint scraper.The combined stripper/paint layer has no lateral cohesion, however, sothat it is necessary to remove it by passing the scraper over the entirearea between the layer and the underlying surface. On flat areas thiscauses no difficulty but over complex curvatures, such as on chair legsor cornices, the removal is not readily accomplished with a scraper.

It has now been found that remarkably good paint stripping may beachieved, particularly on surfaces bearing oil-based paint, with astripper which forms a peelable skin over the painted surface. Thealkali is held in contact with the paint to be stripped which is thusefficiently softened, and the subsequent removal of the stripper/paintlayer is greatly facilitated, particularly on complex shapes.

Accordingly, in one aspect this invention provides a spreadable aqueouspaint stripper composition which comprises an alkali in an amount suchthat the composition is capable of stripping paint, together with one ormore skin forming promoter components whereby, when the composition isapplied as a layer over a painted surface to be stripped, it forms overthe surface a peelable skin, the skin having a tear strength sufficientfor it to be separated and removed from the underlying surface and anadhesive strength sufficient to separate and remove from the underlyingsurface paint softened by contact with the composition.

The composition of the invention is applied as a layer over a paintedsurface to be stripped, and forms a peelable skin thereover. Thepeelable skin has a tear strength sufficient for it to be separated fromthe underlying surface as an integral gel. Thus, the layer ofcomposition develops a lateral strength which permits its removal as anintact skin. However, the lateral strength need not be high for theadvantages to be achieved, and the skin need not, for example, possesslateral elastic properties.

The means for providing the peelable property in the skin formed usingthe compositions and the process of the invention may take a number offorms. In a first embodiment the composition may comprise fibrousmaterial as a skin forming promoter component which bonds together theaqueous alkaline composition to form a peelable skin. Such compositionsgenerally contain a gelling agent as a skin forming promoter componentin addition to the fibre. In a modification of this embodiment thecomposition may contain a filler as an additional skin forming promotercomponent to thicken the composition and increase the absorptiveproperties of the peelable skin.

In an alternative embodiment, no fibrous material is employed, but thepeelable property is achieved by thickening the composition with fillerand employing a particular combination of gelling agents.

Thus in its broadest aspect the first embodiment of the inventionmentioned above is a spreadable aqueous composition comprising an alkaliand an appropriate amount of fibrous material, the composition beingsufficiently alkaline to strip paint.

The alkali is most conveniently an alkali metal hydroxide, since thesegive a highly alkaline composition and are readily available and cheap;sodium and potassium hydroxide are preferred. However, it is possible touse other alkaline materials having a pH in aqueous solution greaterthan 10, such as alkali metal silicates--for example, sodiummetasilicate--and phosphates--for example, trisodium phosphate. Otherhigh alkaline materials, preferably with a pH greater than 12 and mostpreferably with a pH greater than 13, are well known in the art andcould be used in the compositions of the invention. Sodium hydroxide isgenerally most preferred for use as the alkaline material, as othercompounds are more expensive and milder or slower acting. If desired,sodium hydroxide may be provided by using a mixture of sodium carbonateand calcium hydroxide.

In some applications, such as where there may be a danger of stainingthe surface under the paint, it may be desirable to select a milderalkaline agent, and in that case trisodium phosphate might be used.

The concentration of alkaline material in the composition will beselected according to known criteria, and this selection is believed tobe within the competence of one skilled in the art. When sodiumhydroxide is used as the alkaline material, it would be possible toemploy concentrations of 25% by weight or higher. However, it ispreferred to use no more than 12% sodium hydroxide, since this avoidsrestrictions in use resulting from regulations for handling poisons. Thelower concentration limit is determined by the paint stripping job forwhich the composition is intended, and could be as low as 1% or evenlower. However, the usual concentration range for sodium hydroxide isfrom 3% to 12%, and most preferably from 3% to 7%. Within thosepreferred ranges the particular concentration is selected having regardto the speed of stripping that is required, set against the effect onthe cost of the stripper.

When other alkaline materials are used the preferred concentration wouldbe judged on a similar basis. Potassium hydroxide may be used atconcentrations of up to 17% without requiring special treatment underpoisons regulations, so this alkali might be preferred when a higherconcentration is needed. A likely concentration range for the otheralkaline materials mentioned hereinbefore is from 30% to 40%.

The compositions preferably contain a gelling agent, as an additionalskin forming promoter component, and this may be a natural or synthetic,water-soluble polymer. Such gelling agents are well known to thoseskilled in the art. Examples of appropriate agents are polyacrylic acidand its lower-alkyl esters (where the lower-alkyl group contains fromone to four carbon atoms), starch, natural gums such as gum tragacanthor British gum, cellulose ethers, hydroxyethyl cellulose, xanthan gumsand synthetic clays such as montmorillonite clays. Preferred gellingagents are the xanthan gums and synthetic clays. An example of acommercially available xanthan gum is Kelzan, a high molecular weightnatural polysaccharide produced in a pure culture fermentation processby the microorganism Xanthomonas campestris, and available from Kelco, adivision of Merck and Co. Inc. A preferred synthetic clay is Laponite,which is a synthetic hectorite--a magnesium silicate containing lithium,and fluorine or hydroxide groups in a layered structure. This clay isavailable from Laporte Industries Limited.

The amount of gelling agent used is selected on the basis of thethickness required for the composition having regard to the propertiesof the particular gelling agent used. It is not possible to indicatepreferred ranges for gelling agents in general, since different agentsare used at different concentrations to achieve the same degree ofgelling. However, it is believed to be within the competence of oneskilled in the art to select an appropriate gelling agent concentration.By way of illustration only, it may be said that it is unlikely that thegelling agent will be used in concentrations in excess of 10% by weight,while a convenient lower limit is 0.1%. Examples are given hereinafterwhich further illustrate suitable concentrations of gelling agents inparticular compositions.

The fibrous material in the first embodiments of the invention functionsto hold together the gelled layer of composition on the painted surfaceand so make the layer peelable. A wide variety of organic and inorganic,natural and synthetic fibres may be used to contribute peelability tothe composition, provided that the fibre is not degraded by the othercomponents of the composition at the concentrations employed. The fibresmay be, for example, ground paper, flaked paper, paper punchings, wool,textile fibres, asbestos, wood pulp, or plastics material fibre such asviscose fibre (for example rayon or cellophane fibres) or groundsynthetic leather (for example polyvinyl or polyvinyl chloride fibres).These forms of fibre are relatively cheap since they are derived fromwaste materials, and ground paper in particular provides a cheap sourceof fibre and for that reason is preferred.

The fibre used is preferably in the form of relatively short individualfibres, by which it is meant that the fibres do not exceed 10 mm. inlength. A preferred form of random ground paper fibre has fibres withlengths of up to 2 mm.

The fibre content of the composition is chosen to give the desireddegree of peelability in conjunction with the selected gelling agent.Typically the fibre will constitute no more than 25% by weight of thecomposition, and in general compositions will contain no more than 15%fibre and preferably no more than 12% fibre. Too much fibre may lead tothe composition being too thick for it to be applied conveniently to thesurface being stripped. A preferred lower limit is about 5% fibre, whileexcellent peelability is achieved with fibre contents of not less than8%.

The fibre-containing compositions of the invention may also contain afiller--that is to say, a solid, particulate component substantiallyinert to the other components of the composition and to paint, again asan additional skin forming promoter component. This filler acts to bulkout the composition and increase its thickness. In addition, when thecomposition is in use the filler absorbs the liquified paint which hasbeen attacked by the alkali, and helps to prevent runs. Suitable fillerswhich may be used include powdered chalk, clay, fly ash, vermiculite andwood flour. Of these, powdered chalk is most preferred for use in theinvention.

The filler should not be employed in a concentration at which thecomposition becomes too thick to be spreadable, and clearly the amountof filler which may be employed to give a spreadable composition isaffected by the amount of fibre present. In general, when filler ispresent the amount of fibre may be reduced. It has been found desirablefor up to 64% filler to be used in conjunction with up to 20% fibre, andeffective compositions may be obtained with filler and fibre even whenthe amount of fibre is below 5%, which is the preferred lower limit forfibre content in the absence of filler. It has been found that the ratioof filler to fibre is most conveniently in the range of from 6:1 to 1:1,and most preferably in the region of 4:1.

It has also been found that peelable compositions may be obtainedwithout the use of fibre, by using filler in conjunction with a specificcombination of gelling agents. Thus in the alternative embodiment, thisinvention provides a spreadable composition comprising an alkali,filler, a xanthan gum and a synthetic hectorite clay, the compositionbeing sufficiently alkaline to be capable of stripping paint.

The alkali used in this embodiment is selected according to the criteriaset out hereinbefore and employed at the concentrations previouslysuggested.

The filler may be selected from those materials identified hereinbefore,but is generally used at somewhat higher concentrations. The filler willgenerally be present in an amount of at least 25% by weight, andpreferably of up to 75% by weight. Most preferably the amount of fillerlies in the range of from 50% to 70% in the fibre-free compositions. Theactual amount of filler used is determined by the rheological propertiesof the compositions, which are in turn set by the nature andconcentrations of the gelling agent. In qualitative terms, the amount offiller should not be so low as to result in a slurry which would tend torun off a surface being stripped. Neither should so much filler bepresent that the composition cannot readily be spread onto a surface.

In a preferred embodiment the composition is thixotropic, but naturallyeven when it exhibits this property it should become sufficiently mobileon being worked to permit spreading.

It has been found that a particular combination of gelling agents isrequired to obtain peelability in a composition containing filler and nofibre. The xanthan gum is preferably Kelzan gum, and the clay ispreferably Laponite. These gelling agents are preferably used atconcentrations of from 0.2% to 4% Kelzan and from 0.1% to 2.0% Laponite,the ratio of Kelzan to Laponite being in the range of from 5:1 to 1:1.

Both types of compositions of the invention, and thus whether or notcontaining fibre, may also include other standard ingredients for paintstripping compositions such as wetting agents, dyes, preservatives andwater-miscible organic solvents. All such additives must of course becompatible with the other ingredients, and in particular they must becompatible with the alkalinity of the compositions, as well as beingwater-miscible. The selection of these conventional ingredients andsuitable concentrations for them is believed to be within the competenceof one skilled in the art.

In use the compositions of the invention are applied to a paintedsurface typically as a layer of up to 3-mm. thick. It is convenient toapply the composition to the surface with a trowel or similar spreader.The composition is then left until the paint has been softened;typically this will be a period of from one to six hours, althoughlonger periods of contact may be desirable when many coats of paint areto be removed. If the layer of composition dries out during this time,it can be sprayed with water to dampen it and ensure maximum contact ofthe alkali with the paint.

The layer of composition is then removed as a peelable skin using ascraper or similar tool, although actual scraping of the underlyingsurface should not be necessary. The skin is lifted away from thesurface together with the softened paint, and complex shapes can bereadily stripped without having to scrape paint from every crevice.

In a second aspect, the invention thus includes a method of stripping apaint layer from an underlying surface, which method comprises applyingto the paint layer an effective amount of a composition as describedherein, allowing the composition to form a peelable skin in conjunctionwith paint softened by the action of the composition, lifting the formedskin and peeling it away from the underlying surface to strip the painttherefrom.

The compositions of the invention have been described above in theirready-to-use form. It is to be understood, however, that thecompositions of the invention may be produced for distribution and salein dry powder form. Accordingly, the present invention in another aspectprovides a composition in dry powder form which, when mixed with water,can form a spreadable aqueous paint stripper composition as describedherein.

The dry powder composition may be the same as the spreadable aqueouscomposition, except for the inclusion of water. However, since at leastsome of the alkali materials described above may not be convenientlyhandled in dry form, it is preferred to formulate the dry compositionwith a mixture of calcium hydroxide (lime) and an alkali metal salt, theanion of which forms an insoluble salt with calcium. The ingredients ofsuch a mixture when mixed with water provide an alkali metal hydroxide,e.g. sodium hydroxide, but when in dry form are more easily handled thanthe corresponding alkali metal hydroxide.

Preferably, the alkali metal salt and the calcium hydroxide are employedin substantially stoichiometric amounts, the percentage of each in thedry composition being such as to provide a corresponding percentage ofalkali metal hydroxide in accordance with the above description,preferably 3 to 12% by weight of hydroxide.

Since the dry powder composition is especially useful as ado-it-yourself product, it may if necessary be formulated with anincreased amount of filler (where present) to facilitate mixing withwater. Thus, for example, where a filler and fibre together are employedthe ratio of filler to fibre may be up to about 7:1.

Preferably the dry powder composition includes a mixture of lime andsodium carbonate to provide sodium hydroxide when mixed with water.

The following Examples are now given, though only by way ofillustration, to show aspects of the invention in more detail.

EXAMPLE 1 Paint stripper containing fibre and filler

A composition was prepared from the following ingredients:

    ______________________________________                                                              Grams                                                   ______________________________________                                        Kelzan                  26.2                                                  Sodium hydroxide solution (46% w/w)                                                                   104.6                                                 Ground paper            104.6                                                 Chalk                   104.6                                                 Water                   660.0                                                                         1000.0  grams                                         ______________________________________                                    

EXAMPLE 2 Paint stripper containing fibre

A composition was prepared from the following ingredients:

    ______________________________________                                                              Grams                                                   ______________________________________                                        Kelzan                  26.2                                                  Laponite S              10.7                                                  Sodium hydroxide solution (46% w/w)                                                                   104.6                                                 Ground paper            104.6                                                 Water                   753.9                                                                         1000.0  grams                                         ______________________________________                                    

EXAMPLE 3 Paint stripper containing filler

A composition was prepared from the following ingredients:

    ______________________________________                                                              Grams                                                   ______________________________________                                        Kelzan                  10.5                                                  Laponite S              4.3                                                   Sodium hydroxide solution (46% w/w)                                                                   83.3                                                  Chalk                   640.0                                                 Water                   261.9                                                                         1000.0  grams                                         ______________________________________                                    

EXAMPLE 4 Paint stripper containing filler and fibre

A thixotropic composition was prepared from the following ingredients:

    ______________________________________                                                               kg.                                                    ______________________________________                                        Kelzan                   1.00                                                 Laponite S               0.50                                                 Sodium hydroxide solution (46% w/w)                                                                    10.45                                                Chalk                    34.83                                                1/16" Ground paper       7.46                                                 Water                    45.77                                                                         100.01  kg.                                          ______________________________________                                    

EXAMPLE 5 Paint stripper containing filler and fibre

A composition was prepared from the following ingredients:

    ______________________________________                                                                kg.                                                   ______________________________________                                        Kelzan                    0.871                                               Laponite S                0.435                                               Sodium hydroxide solution (46% w/w)                                                                     10.45                                               Chalk                     37.31                                               1/8" Flaked paper         8.71                                                Water                     42.226                                                                        100.00  kg.                                         ______________________________________                                    

EXAMPLE 6 Paint stripper containing filler and fibre

A composition was prepared from the following ingredients:

    ______________________________________                                                       % w/w                                                          ______________________________________                                        Kelzan            2.6                                                         Sodium hydroxide  5.0                                                         1/8" Ground paper                                                                              10.5                                                         300 Mesh wood flour                                                                            10.5                                                         Water            71.4                                                         ______________________________________                                    

EXAMPLE 7 Paint stripper containing filler and fibre

A composition was prepared from the following ingredients:

    ______________________________________                                                        % w/w                                                         ______________________________________                                        Kelzan             2.6                                                        Sodium hydroxide   5.0                                                        500 Mesh vermiculite                                                                            10.5                                                        1/8" Ground paper 10.5                                                        Water             71.4                                                        ______________________________________                                    

EXAMPLE 8 Paint stripper containing filler and fibre

A composition was prepared from the following ingredients:

    ______________________________________                                                        % w/w                                                         ______________________________________                                        Kelzan            2.0                                                         Laponite          0.8                                                         Sodium hydroxide  5.0                                                         Chalk             21.0                                                        500 Mesh vermiculite                                                                            10.5                                                        1/16 ' "paper     5.0                                                         Water             55.7                                                        ______________________________________                                    

EXAMPLE 9 Paint stripper containing filler and fibre

A composition was prepared from the following ingredients:

    ______________________________________                                                       % w/w                                                          ______________________________________                                        Kelzan           1.0                                                          Laponite         0.5                                                          Potassium hydroxide                                                                            5.0                                                          Chalk            35.0                                                         1/8" Flaked paper                                                                              7.5                                                          Water            51.0                                                         ______________________________________                                    

EXAMPLE 10 Paint stripper containing filler and fibre

A composition was prepared from the following ingredients:

    ______________________________________                                                       % w/w                                                          ______________________________________                                        Kelzan            2.6                                                         Sodium hydroxide  5.0                                                         Chalk            10.5                                                         Viscose fibres   10.5                                                         Water            71.4                                                         ______________________________________                                    

EXAMPLE 11 Paint stripper containing fibre and filler

A composition was prepared from the following ingredients:

    ______________________________________                                                           % w/w                                                      ______________________________________                                        Water                58.2                                                     Chalk                15.7                                                     1/4" Flaked paper    15.7                                                     Sodium hydroxide (46% w/w)                                                                         10.4                                                                          100.0                                                    ______________________________________                                    

The pH of the compositions of Examples 1 to 11 fall within the range offrom 13 to 14.

EXAMPLE 12 Paint stripper containing fibre and filler

A composition was prepared from the following ingredients:

    ______________________________________                                                        % w/w                                                         ______________________________________                                        Kelzan            1.0                                                         Laponite S        0.5                                                         Trisodium phosphate                                                                             10.0                                                        Chalk             34.8                                                        1/16" Ground paper                                                                              7.5                                                         Water             46.2                                                        The pH of this composition was 11.8.                                          ______________________________________                                    

EXAMPLE 13 Paint stripper containing fibre and filler

A composition was prepared from the following ingredients:

    ______________________________________                                                       % w/w                                                          ______________________________________                                        Kelzan           1.0                                                          Laponite S       0.5                                                          Sodium metasilicate                                                                            10.0                                                         Chalk            34.8                                                         1/8" Flaked paper                                                                              7.5                                                          Water            46.2                                                         ______________________________________                                         The pH of this composition was 12.7                                      

EXAMPLE 14 Paint stripper containing fibre and filler in dry powder form

A dry powder composition was prepared from the following ingredients:

    ______________________________________                                                      Parts by weight                                                 ______________________________________                                        Sodium carbonate                                                                              5.0                                                           Calcium hydroxide                                                                             3.5                                                           Chalk           34.0                                                          Granulated paper                                                                              5.0                                                           1/8" (3.2 mm)                                                                 Kelzan          0.75                                                          Laponite S      0.38                                                          ______________________________________                                    

The above composition was suitable for transport, storage and sale indry powder form and, when mixed with water to 100 parts w/w, formed aspreadable aqueous paint stripper composition which in use formed apeelable skin and afforded 100% paint removal without scraping as didthose compositions containing sodium hydroxide.

EXAMPLE 15 Paint strippers containing fibre and filler in dry powderform

The compositions described in Examples 12 and 13 were formulated withoutwater in dry powder form. When required for use the compositions weremixed with water to 100% w/w and performed in the manner describedabove.

The following Test Results are now presented, again only by way ofillustration, to show the results obtained when stripping paint with aprior art composition and with the compositions of the invention.

Test Results

Method

Paint stripping compositions were tested for their efficacy in removingfour layers of oil-based paint from a wood surface. The painted surfacewas divided into test areas approximately 6 inches (15 cm)×4 inches (10cm) using pieces of self-adhesive plastics tape. A paint strippercomposition under test was applied to a test area using a trowel orspatula to a thickness of approximately 1/8 inch (3 mm) judged by eye.The composition was left in contact with the test area for a period oftime and then removed using a spatula. The nature of the composition atthe time of removal, the ease of removal and the amount of paintstripped was assessed. The results are set out hereinafter.

Results

To provide a comparison, a prior art paint stripper was tested. Thecomposition of this prior art stripper was:

    ______________________________________                                                            % (w/w)                                                   ______________________________________                                        Gelling agent (Viscalex HV30 - an                                                                   3.0                                                     acrylic copolymer emulsion avail-                                             able from Allied Colloids Ltd)                                                Sodium hydroxide      4.0                                                     Chalk                 66.5                                                    Anti-foaming agent    0.2                                                     Water                 26.2                                                    ______________________________________                                    

In separate tests this prior art composition was left in contact withtest areas for 2/3 hour and 2 hours. The results are given in Table 1below.

                  TABLE 1                                                         ______________________________________                                                                  Nature at                                                                              Paint                                      Composition                                                                             Contact time (hours)                                                                          removal  stripped                                   ______________________________________                                        Prior art 2/3             Paste     80%                                       Prior art 2               Paste    100%                                       ______________________________________                                    

In each case the paint stripper composition was removed as a paste; itshowed no cohesion and was not peelable. As a result it was necessary toscrape the paste off the stripped surface, which was a messy andtime-consuming operation. After 2/3 hour, 80% removal was achieved--thatis to say, all four layers of paint were removed over 80% of the testarea. After 2 hours, 100% removal was achieved, and all four layers ofpaint were stripped from the test area.

The results obtained in testing compositions of the invention are setout in Table 2 below, in which the compositions are identified byreference to the Example in wich the formulation details are given.

                  TABLE 2                                                         ______________________________________                                                   Contact time                                                                             Nature at    Paint                                      Composition                                                                              (hours)    removal      stripped                                   ______________________________________                                        1          1          Peelable skin                                                                              100%                                       2          1          "            100%                                       3          1          "            100%                                       4          1          "            100%                                       4          2/3        "            100%                                       5          1          "            100%                                       6          1          "            100%                                       7          1          "            100%                                       8          1          "            100%                                       9          1          "            100%                                       10         1          "            100%                                       11         10         Dry peelable skin                                                                          100%                                       12         5          Peelable skin                                                                              *                                          13         5          "            **                                         ______________________________________                                         *One layer completely removed, a second layer removable with scraping         **Approximately 90% removal with screening                               

In each case the composition tested formed a peelable skin which couldbe lifted away from the test area in one piece together with the paintsoftened by the alkali. With the compositions containing sodiumhydroxide or potassium hydroxide, 100% removal was obtained withoutscraping; it was simply necessary to wash down the stripped area afterremoving the skin to leave a clean stripped wood surface. Thecompositions containing trisodium phosphate and sodium metasilicate areintended to be milder so that they may be used on delicate workpieces orwhere selective removal of only top layers of paint is required. Thus inthe test, complete removal of all four layers of paint was not achievedwith these alkaline materials, but the compositions displayed theadvantage of the invention in that they formed a peelable skinfacilitating the stripping operation.

We claim:
 1. A composition in dry powder form which can be mixed withwater to form a spreadable aqueous paint stripper, which comprisescalcium hydroxide and an alkali metal salt, the anion of the alkalimetal salt being one which forms a water-insoluble salt with calcium, tothereby provide alkali in an amount such that the aqueous stripper iscapable of stripping paint, together with one or more skin formingpromoter components comprising either a natural or syntheticwater-soluble polymer as a gelling agent in combination with a filler,or a fibrous material in the form of relatively short individual fiberswhereby, when the aqueous stripper is applied as a layer over a paintedsurface to be stripped, it forms over the surface a peelable skin, theskin having a tear strength sufficient for it to be separated andremoved from the underlying surface and an adhesive strength sufficientto separate and remove the underlying surface paint softened by contactwith the composition.
 2. A composition according to claim 1 wherein theskin forming promoter component comprises a fibrous material.
 3. Acomposition according to claim 1 wherein the skin forming promotercomponent comprises a combination of xanthan gum and hectorite clay. 4.A composition according to claim 2 wherein the alkali metal salt issodium carbonate.
 5. A composition according to claim 3 wherein thealkali metal salt is sodium carbonate.
 6. A composition according toclaim 4 wherein the calcium hydroxide and the sodium carbonate togetherprovide from 3% to 12% by weight of sodium hydroxide in the aqueousstripper.
 7. A composition according to claim 5 wherein the calciumhydroxide and the sodium carbonate together provide from 3% to 12% byweight of sodium hydroxide in the aqueous stripper.
 8. A compositionaccording to claim 5 when mixed with water.
 9. A composition accordingto claim 7 when mixed with water.
 10. A method of stripping a paintlayer from an underlying surface which comprises applying to the paintlayer an effective amount of a composition as claimed in claim 1,allowing the composition to form a peelable skin in conjunction withpaint softened by the action of the composition, lifting the formed skinand peeling it away from the underlying surface to strip the painttherefrom.
 11. A method of stripping a paint layer from an underlyingsurface, which comprises applying to the paint layer an effective amountof a composition as claimed in claim 8, allowing the composition to forma peelable skin in conjunction with paint softened by the action of thecomposition, lifting the formed skin and peeling it away from theunderlying surface to strip the paint therefrom.
 12. A method ofstripping a paint layer from an underlying surface, which comprisesapplying to the paint layer an effective amount of a composition asclaimed in claim 9, allowing the composition to form a peelable skin inconjunction with paint softened by the action of the composition,lifting the formed skin and peeling it away from the underlying surfaceto strip the paint therefrom.